New research finds moderate cognitive impairments in heavy cannabis users

People diagnosed with cannabis use disorder tend to perform more poorly on a range of cognitive tasks—even after the immediate effects of the drug have worn off—according to a new meta-analysis published in the journal Addictive Behaviors. The findings indicate that individuals with cannabis use disorder tend to have moderate impairments in verbal learning and memory, working memory, and processing speed. While smaller effects were observed in attention and verbal fluency, the results provide evidence that the cognitive impacts of heavy cannabis use may be more substantial than what has been found among recreational users.

As cannabis becomes legal in more jurisdictions for both recreational and medical use, researchers and public health officials are trying to better understand its long-term effects. One area that has drawn particular concern is whether chronic use of cannabis may lead to lasting impairments in cognitive functioning. While prior studies have confirmed that cannabis can impair memory and attention during acute intoxication, the picture has been less clear when it comes to more persistent effects after the drug wears off.

Much of the existing research has focused on people who use cannabis recreationally, often without distinguishing between occasional users and those with cannabis use disorder. This distinction matters because, for other substances such as alcohol or cocaine, research has shown that cognitive deficits tend to be larger among individuals who meet clinical criteria for substance use disorder.

“When examining the recreational use of cannabis, some previous investigators were led to conclude that the effects of cannabis on cognition are minimal. We thought it would be more relevant, from a clinical point of view, to focus on the cognitive deficits of people with cannabis use disorder more specifically. So we performed a quantitative synthesis (e.g. meta-analysis) of all the studies available on the topic,” said study author Stéphane Potvin, a professor of psychiatry at the University of Montreal.

A meta-analysis is a method that combines the results of multiple studies to draw broader conclusions. Meta-analyses are especially helpful when individual studies yield mixed or inconsistent results, as they allow researchers to pool data and identify patterns across a larger body of evidence.

To be included in the meta-analysis in the new research, studies had to meet several criteria. Each study needed to compare people diagnosed with cannabis use disorder to a control group of healthy individuals. The studies also had to use validated neuropsychological tests to assess cognitive functioning and could not focus solely on the immediate effects of cannabis intoxication. The researchers excluded studies where participants had other substance use disorders or psychiatric diagnoses that might confound the results.

After an extensive literature search across multiple databases, the researchers identified 23 studies that met their inclusion criteria. These studies collectively included data from over 100 separate cognitive test results, grouped into 13 different domains, such as verbal memory, working memory, attention, executive functioning, and visual memory.

The analysis used a multivariate statistical approach to estimate the overall effect size, a standardized measure of how strongly cannabis use disorder was associated with poorer cognitive performance. They also conducted sensitivity analyses to ensure that no single study was unduly influencing the results. In addition, they examined whether variables such as age, gender, frequency of cannabis use, or duration of abstinence influenced cognitive performance.

Across all studies, individuals with cannabis use disorder consistently performed worse on cognitive tasks compared to healthy controls. The overall effect size for general cognitive impairment was in the small-to-moderate range (Cohen’s d = 0.35), suggesting a meaningful difference in performance that is unlikely to be explained by chance.

The analysis of specific cognitive domains showed that certain types of mental functions were more affected than others. The largest deficits were found in verbal learning and memory, working memory, processing speed, and general intelligence measures. For these areas, effect sizes approached or exceeded d = 0.4, which is typically considered a moderate effect.

“Cannabis use disorder is associated with significant deficits in cognitive domains like verbal learning, verbal memory, speed of processing and working memory,” Potvin told PsyPost. “These cognitive domains are important in that they allow us to interact with people and to function on a daily basis.”

“The cognitive domains that we found to be impaired in people with cannabis use disorder (who are not intoxicated at the moment of being tested) are pretty much the same as the cognitive domains found to be impaired in healthy participants tested in an acute state of THC (tetrahydrocannabinol) intoxication. In my opinion, the overall coherence of these results increase the confidence we should have in them.”

In contrast, smaller differences were observed in tasks related to attention, verbal fluency, and impulsivity, where effect sizes ranged from d = 0.25 to 0.28. The researchers noted that while these smaller effects may not be as noticeable in day-to-day functioning, they still indicate a consistent pattern of reduced performance.

Interestingly, the researchers found no clear link between the severity of cognitive impairments and various background factors. Meta-regression analyses showed that age, sex, frequency or duration of cannabis use, and the length of abstinence before testing did not significantly influence the results. The authors cautioned, however, that most of the included studies involved participants who had abstained from cannabis for 10 days or less, making it difficult to evaluate longer-term recovery effects.

Although this meta-analysis improves upon previous reviews by focusing specifically on cannabis use disorder and including a larger number of studies, it is not without limitations.

Most of the studies included were cross-sectional, which means they only assessed cognitive functioning at a single point in time. This makes it difficult to determine whether the cognitive impairments preceded the onset of cannabis use disorder or were caused by it. Longitudinal studies that follow people over time would be better suited to address questions of causality.

Another limitation is that some cognitive domains were assessed in only a few studies. For example, visual memory and emotion recognition were examined in fewer than five studies, making it hard to draw firm conclusions about these areas.

The apparent lack of effect from abstinence duration may also reflect a gap in the research. Few studies included participants who had been abstinent for longer than a month, so it remains unclear whether some cognitive functions might recover with sustained abstinence.

“I see two main questions unanswered,” Potvin said. “First, we need to determine how long-lasting are the cognitive impairments seen in people with cannabis use disorder. Second, we need to know if these deficits are more prominent in people who use very high potency cannabis products, such as some cannabis concentrates.”

The study, “The effects of cannabis use disorder on cognitive functions: A meta-analysis,” was authored by Florence Pilon, Alexandre Dumais, Charles-Édouard Giguère, and Stéphane Potvin.